vSphere Storage - ESXi 5.1 - Documentation - VMware

n You cannot convert larger than 2TB RDMs to virtual disks, or perform other operations that involve RDM
to virtual disk conversion. Such operations include cloning.
Dynamic Name Resolution
The RDM file supports dynamic name resolution when a path to a raw device changes.
VMFS uniquely identifies all mapped storage devices, and the identification is stored in its internal data
structures. Any change in the path to a raw device, such as a Fibre Channel switch failure or the addition of a
new HBA, can change the device name. Dynamic name resolution resolves these changes and automatically
associates the original device with its new name.
Raw Device Mapping with Virtual Machine Clusters
Use an RDM with virtual machine clusters that need to access the same raw LUN for failover scenarios. The
setup is similar to that of a virtual machine cluster that accesses the same virtual disk file, but an RDM replaces
the virtual disk file.
Figure 18-3. Access from Clustered Virtual Machines
Host 3
VM3
“shared” access
mapping file
VMFS volume
Host 4
VM4
address
resolutiion
mapped
device
Comparing Available SCSI Device Access Modes
The ways of accessing a SCSI-based storage device include a virtual disk file on a VMFS datastore, virtual mode
RDM, and physical mode RDM.
To help you choose among the available access modes for SCSI devices, the following table provides a quick
comparison of features available with the different modes.
Table 18-1. Features Available with Virtual Disks and Raw Device Mappings
ESXi Features Virtual Disk File Virtual Mode RDM Physical Mode RDM
SCSI Commands Passed
Through
No No Yes
vCenter Server Support Yes Yes Yes
Snapshots Yes Yes No
Distributed Locking Yes Yes Yes
Chapter 18 Raw Device Mapping
REPORT LUNs is not passed
through
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